1. Field of the Invention
The present invention relates to a technological field of image forming apparatuses, particularly to a process in the case of a document formed with a tint block.
2. Description of the Related Art
There have been conventionally proposed various technologies for preventing or inhibiting copying and duplication of confidential documents needed to be kept confidential such as data including confidential information of companies or utilization of copied media. One of these technologies is a tint block printing technology for a tint block including a background part formed by a certain number of screen lines and a latent image part formed by a larger number of screen lines (halftone dot density) than the former screen lines in a document image.
If a document image generated using this tint block printing technology is copied, an area corresponding to the latent image part of the document image becomes white in the copied media, wherefore a pattern such as a “Copying Prohibited” visually comes up to be seen. This can psychologically restrict or inhibit the copying of the document.
On the other hand, there is a widely known technology, separately from the above tint block printing technology, for reading a document image, detecting a special dot pattern embedded in a background image included in an image data obtained by the reading, comparing the detected special dot pattern with a special dot pattern stored beforehand and prohibiting an image forming operation on a sheet if the both patterns coincide.
There is also known an image forming apparatus (1) for prohibiting a copying operation by detecting a pattern image indicating a copy prohibit code using a copy prohibit information detector when such a pattern image is included in a document image read by an image reader while printing the read image on a sheet when the pattern image indicating the copy prohibit code is not included and (2) for detecting a pattern image indicating a condition code using a condition information detector when such a pattern image is included in a document image read by the image reader, permitting the copying operation only when the detected pattern image agrees with a specified condition registered beforehand while outputting the read image on a sheet when the pattern image indicating the condition code is not included.
In recent years, there has been also known a technology for forming a chromatic tint block on a document and, using this technology, chromatic tint blocks are formed on documents in some cases. Here, there has been a room for reducing the consumption of developers (toners) in the case of copying a document of this type.